Aircraft can and do overrun the ends of runways, raising the possibility of injury to passengers and destruction of or severe damage to the aircraft. Such overruns have occurred during aborted take-offs or while landing, with the aircraft traveling at speeds up to 80 knots. In order to minimize the hazards of overruns, the Federal Aviation Administration (FAA) generally requires a safety area of one thousand feet in length beyond the end of the runway. Although this safety area is now an FAA standard, many runways across the country were constructed prior to adoption of this standard. These runways may be situated such that water, roadways, or other obstacles prevent economical compliance with the one thousand foot overrun requirement.
In order to alleviate the severe consequences of overrun situations, several materials, including existing soil surfaces beyond the runway, have been assessed for their ability to decelerate aircraft. However, soil surfaces are not the best solution for arresting moving vehicles (i.e. aircraft), primarily because their properties are unpredictable.
Another system that has been explored is providing a vehicle arresting system or other compressible system that includes material or a barrier placed at the end of a runway that will predictably and reliably crush (or otherwise deform) under the pressure of aircraft wheels traveling off the end of the runway. The resistance provided by the compressible, low-strength material decelerates the aircraft and brings it to a stop within the confines of the overrun area. Specific examples of vehicle arresting systems are called Engineered Materials Arresting Systems (EMAS), and are now part of the U.S. airport design standards described in FAA Advisory Circular 150/5220-22B “Engineered Materials Arresting Systems (EMAS) for Aircraft Overruns” dated September 2012. EMAS and Runway Safety Area planning is guided by FAA Orders 5200.8 and 5200.9.
A compressible (or deformable) vehicle arresting system may also be placed on or in a roadway or pedestrian walkway (or elsewhere), for example, for purposes of decelerating vehicles or objects other than aircraft. The systems may be used to safely stop cars, trains, trucks, motorcycles, tractors, mopeds, bicycles, boats, or any other vehicles that may gain speed and careen out of control, and thus need to be safely stopped.
Some specific materials that have been considered for arresting vehicles (particularly in relation to arresting aircraft), include cellular concrete, foamed glass, ultra lightweight cementitious materials, perlite and cement, and chemically bonded phosphate ceramic (CBPC). These materials can be formed as a shallow bed in an arrestor zone at the end of the runway. When a vehicle enters the arrestor zone, its wheels may apply pressure to the material, causing the material to crush or collapse and create an increase in drag load.
However, some of the materials that have been explored to date can be improved upon. For example, some types of foams can be disadvantageous in that they may have an immediate “rebound” characteristic, resulting in return of some energy following compression. For example, they may snap back to shape, much like a rubber band. This is undesirable because once the arresting function is complete, extraneous energy should not then be re-asserted against the aircraft tire. (However, as described below, some of the materials according to this disclosure may have a slow rebound characteristic, such that any rebound that may occur takes place after the vehicle has passed the system.) Cellular concrete has density and compressive strength properties that may vary with time and that some properties could be difficult to maintain in production due to the innate properties of its variable raw materials and subsequent hydration process. A ceramic property is set to a stable state during firing process, but foamed glass properties can be difficult to control (including uniformity, particle size, grain strength, etc). It is thus desirable to develop improved materials for vehicle arresting beds.
One further example has explored an aircraft arresting system using phenolic foam panels. This is outlined in U.S. Pat. No. 5,193,764. This solution seeks to provide rigid foam boards that can be stacked in layers and secured to one another via adhesive. The rigid foam is a closed cell structure, having a density in the range of 2-4 pounds per cubic foot and a compressive strength in the range of 20-80 pounds per square inch. The preferred material described in this patent is phenolic foam, and the adhesive is a latex adhesive. Phenolic foam on its own has high water absorption, so this system was not explored or pursued further by the FAA.
ACRP Report 29—“Developing Improved Civil Aircraft Arresting Systems” is a 2009 report created by the Transportation Research Board (and sponsored by the Federal Aviation Administration). This report explores various materials that may be used as energy absorbing for EMAS. The report mentions polymer cellular foams, such as phenolic foam and styrofoam, but states that “it was determined that a cementitious foam provided advantages over polymer foams” (see Section 2.2), leading one of ordinary skill in the art away from pursuing polymer foams.